March 30, 2012 -- Women giving birth can expect to have longer labors than women did 50 years ago, according to a new analysis by National Institutes of Health researchers.

For first-time mothers, the first stage of labor has increased by 2.6 hours, says Katherine Laughon, MD, a researcher at the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

Women who were having their second or later child took two hours longer in recent years than women in the 1960s, she also found.

Women giving birth in the ‘60s had a median labor time of just under four hours (half took more time, half less). Those in the recent group took 6.5 hours, Laughon says.

She discussed the findings at a news briefing today. The study is published online in the American Journal of Obstetrics and Gynecology.

"We don't have a complete explanation," Laughon says of the longer labors. However, changes in delivery practices appear to be the main contributing factor.

Longer Labor Study: More Findings

Laughon and her colleagues compared nearly 40,000 babies born from 1959 to 1966 with more than 98,000 born between 2002 and 2008.

Cesarean delivery was more common in the recent group. While 12% of the modern mothers had a C-section, 3% of the previous group did.

The mothers in the recent group were older; nearly 27 years old. The women in the previous group were on average 24. The recent mothers were also heavier, with a pre-pregnant body mass index or BMI of 24.6 compared to 22.6. (A BMI of 18.5-24.9 is considered normal weight.)

Babies of the women in the recent group weighed about 3.5 ounces more, on average.

Women in the 1960s group were less likely to have an epidural for pain relief. More than half of the modern mothers got the epidural injection, but just 4% of the mothers giving birth in the 1950s and '60s did.

While epidural use is known to increase delivery times, Laughon says it can't explain all of the increase.

The use of the hormone oxytocin, which speeds labor, was given in 31% of the recent deliveries but just in 12% of the earlier group. Laughon says that without this hormone, the modern labors may have been even longer.

Other practices common in the ‘60s, such as using forceps to help extract the baby, are much less common now, Laughon says.

Longer Labors: Perspective

The findings about longer labors are not at all surprising, says Michael Cabbad, MD, chairman and chief of maternal and fetal medicine at the Brooklyn Hospital Medical Center. He reviewed the findings for WebMD but was not involved in the study.

He agrees that delivery practices help explain the findings. "When patients arrive in active labor, they are placed on monitoring devices that people didn't have in the ‘60s. They tend to be placed in a labor bed. They get IV fluid hydration, which tends to slow labor. Women are in bed with limited mobility, not walking." That also tends to slow down the labor process, he says.

While the study did not look at anxiety levels of the women, Cabbad speculates that could play a role. "Talk to a woman today who is pregnant and about to have a baby," he says. "She gets on the Internet [to research pregnancy and childbirth]."

"This amount of communication and understanding about the birth process is not where it was 50 years ago," Cabbad says. "People were more relaxed about labor and the process of childbirth 50 years ago."

On the other hand, the technology has produced safer labors, he says.

His best advice? "Find an [ob-gyn] you feel comfortable with, that you can communicate with, and that you feel safe with. That relaxes the psyche, so maybe labor will be a little better."